Objective: The purpose of this study was to investigate the vascular function of pregnant women with insulin-dependent diabetes mellitus, using a noninvasive method. Study Design: This was a cross-sectional study that examined vascular function, which was assessed by flow-mediated dilatation of the brachial artery, in 37 pregnant women with insulin-dependent diabetes mellitus and in 37 healthy pregnant women at 20 weeks of gestation. The control of diabetes was also assessed by the measurement of glycosylated hemoglobin. Data were analyzed by 2-sided unpaired t test and multivariate regression analysis.Results: In the pregnant women with insulin-dependent diabetes mellitus, flow-mediated dilatation of the brachial artery was significantly lower than in healthy pregnant women (6.43% ± 3.66% vs 9.43% ± 3.69%, respectively; P =.0008). This difference was apparent even after an adjustment was made for blood vessel diameter, which was different between the 2 populations (P =.01). Flow-mediated dilatation in diabetic women was significantly correlated with the duration of diabetes (P =.01) but not with the levels of glycosylated hemoglobin. Conclusion: Maternal insulin-dependent diabetes mellitus is associated with an impaired vasodilatory response to a blood flow stimulus. This vascular dysfunction is associated with the duration of the diabetes.(Am J Obstet Gynecol 2002;186:84-8.)